Defect inspection method and defect inspection device

ABSTRACT

A defect inspection method and device for irradiating a linear region on a surface-patterned sample mounted on a table, with illumination light from an inclined direction to the sample, next detecting in each of a plurality of directions an image of the light scattered from the sample irradiated with the illumination light, then processing signals obtained by the detection of the images of the scattered light, and thereby detecting a defect present on the sample; wherein the step of detecting the scattered light image in the plural directions is performed through oval shaped lenses in which elevation angles of the optical axes thereof are different from each other, within one plane perpendicular to a plane formed by the normal to the surface of the table on which to mount the sample and the longitudinal direction of the linear region irradiated with the irradiation light.

BACKGROUND

The present invention relates generally to inspection techniques thatuse optically acquired image signal information to detect defectspresent on a surface-patterned object to be inspected. Moreparticularly, the invention is directed to an inspection technique fordetecting microscopic defects present on a patterned substrate such as asemiconductor wafer.

In defect inspection of a substrate having patterns formed on a surface(i.e., a patterned substrate), defect detection sensitivity dependsgreatly upon how accurately a defect-diffracted/scattered detection beamof light, or a defect signal, can be detected in distinction frompattern-diffracted/scattered and underlayer-diffracted/scattereddetection beams of light, or background light noise. During theinspection of a semiconductor wafer, in particular, the detection ofeven more microscopic defects is being demanded in response to theprogress of further microstructured patterning, and how accurately avery weak defect signal from a microscopic defect can be extracteddistinctively from background light noise is a big technologicalchallenge associated with defect inspection.

A vertical structure of a patterned substrate, which is an object to beinspected, and the kinds of defects to be detected are described belowper FIG. 2, taking a semiconductor wafer as an example. FIG. 2 usesreference numbers 20 to 35 and 201 to 251 to denote the verticalstructure of the semiconductor wafer, and uses reference numbers 261 to264 to denote the kinds of defects to be detected.

Reference number 20 denotes an element isolation layer, and referencenumber 202 denotes a structure in which, after trenching of a silicon(Si) substrate 201, the trenches are filled in with silicon oxide(SiO₂), which is an insulator, to provide electrical insulatingseparation between transistor elements formed on the wafer. Referencenumber 21 denotes a gate and contact layer, and reference number 211denotes gate electrode portions formed from polysilicon (poly-Si). Thegate electrode portions are greatly influential upon transistorperformance, weighing heavily in defect inspection as well. Referencenumber 212 denotes contact portions.

Each of the contact portions is where a transistor region and anelectrical interconnect layer formed above the transistor region areinterconnected via a metal, such as tungsten (W), that is buried in ahole etched in the insulating film (silicon dioxide: SiO₂). Theinterconnect layers 22 to 25 form a circuit. These layers are eachfilled in with an insulating film such as silicon dioxide (SiO₂).Reference number 22 denotes a first interconnect layer, which includes afirst interconnect portion 221 for planar interconnection. A first viaportion 222 is where the transistor region and an electricalinterconnect layer formed further above the transistor region areinterconnected via a metal buried in a hole etched in an insulating filmsuch as silicon dioxide (SiO₂). Reference number 23 denotes a secondinterconnect layer, which includes a second interconnect portion 231 anda second via portion 232. Similarly, reference number 24 denotes a thirdinterconnect layer, which includes a third interconnect portion 241 anda third via portion 242. Reference number 25 denotes a fourthinterconnect layer, which includes a fourth interconnect portion 251.

The interconnect portion of each interconnect layer is formed from amaterial including a metal such as aluminum (Al) or copper (Cu). Themetal buried in the via portion is formed from tungsten (W), copper(Cu), or the like.

The defects to be detected are, for example, a scratch 261, a shortcircuit 262 and electrical disconnection 264 that are both a patterndefect, and contamination 263.

FIG. 3 is an explanatory diagram of the steps, materials, and typicaldefects of each layer of the semiconductor device shown in FIG. 2. Thelayers of the semiconductor device are formed through various steps.These steps include: the step of depositing the material which forms thelayer; the step of forming a resist pattern by lithography; the step ofremoving the layer-deposited material by etching it along the formedresist pattern; and chemical mechanical polishing (CMP) forplanarization.

The materials used in each layer and each fabrication step of thesemiconductor device are diverse. The kinds of defects to be detectedalso vary from step to step; in the deposition step, they may becontamination, in the lithographic step for pattern formation and in theetching step, they may be contamination and pattern defects, and in theCMP step for polishing, they may be contamination and scratches.

As described per FIGS. 2 and 3, patterns of various shapes and materialsare involved in semiconductor wafer inspection, and defects of variouskinds are detected. Inspection devices are configured so that aplurality of detection parameters can be set to obtain optimal defectdetection sensitivity according to the particular shape and material ofthe pattern or the kind of defect to be detected.

As described in JP-A-1997-304289 and JP-A-2007-524832, for example,semiconductor wafer defect inspection devices of a darkfield opticaltype that are used to inspect defects and contamination present on asubstrate with patterns formed on a surface are constructed toilluminate the substrate from an oblique direction and converge thelight scattered from the defects, instead of converging via an objectivelens the light regularly reflected from the substrate. These inspectiondevices are also configured so that the light diffracted/scattered froma pattern or underlayer formed on the substrate will be converged viathe objective lens, then intensity-reduced by a polarizing filter and/ora spatial filter, and received by a sensor.

With the above configurations, the defect inspection devices of thedarkfield optical type can generate an inspection image with a defectrepresented explicitly as a luminescent spot against a dark background.Therefore, even if image resolution is too high, that is, a sensor pixelsize on the sample substrate surface is too large (but up to 0.3 μm),for a minimum size of defects to be detected, the devices can detectsmaller defects, for example of 0.1 μm or less in diameter. Since defectinspection devices of the darkfield optical type have such a feature,they are widely used as high-speed high-sensitivity inspection deviceson semiconductor device manufacturing lines.

Semiconductor wafer defect inspection devices of the future will berequired to have an ability to detect even more microscopic defects withthe progress of further device-pattern microstructuring. To respond tothis tendency, the optical systems in the patterned-wafer defectinspection devices of the darkfield optical type need to containappropriate measures against the following several problems.

One of the problems is how to augment a detection aperture (numericalaperture: NA) of the optical system to detect more efficiently the veryweak light scattered from microscopic defects. During patterned-waferdefect inspection, however, it is necessary to detect thedefect-scattered light in distinction from the lightdiffracted/scattered from the patterns or underlayer of the wafer. Ifthe detection aperture is merely augmented, although signal intensity ofthe defect-scattered light will be increased, noise components of thelight diffracted/scattered from the patterns or the underlayer will alsoincrease and detection sensitivity of the defect will be difficult toimprove.

To cope with these problems, it is effective to utilize a difference indirectionality between the defect-scattered light and the pattern- orunderlayer-diffracted/scattered light. More specifically, it iseffective to detect scattered light in a widest possible range from aplurality of different directions and conduct defect detection usingscattered-light images obtained. For example, JP-A-1997-304289 (PatentDocument 1) discloses a technique for inspecting defects by detectingscattered light from a plurality of directions. In addition,JP-A-2007-524832 (Patent Document 2) discloses a technique forinspecting defects using the scattered light acquired by a convergingoptical system placed in an upward direction and oblique direction of asubstrate to be inspected. Furthermore, JP-A-2004-177284 (PatentDocument 3) discloses a technique for inspecting defects usingscattered-light images acquired by an imaging optical system placed inan upward direction and oblique direction of a substrate to beinspected.

Furthermore, JP-A-2008-241688 (Patent Document 4) discloses a techniqueused to inspect defects by changing an angle of a reflecting mirrorpositioned between a substrate to be inspected and a detection opticalsystem placed above the substrate, and thereby acquiring images ofscattered light from a plurality of directions.

Furthermore, JP-A-2010-54395 (Patent Document 5) discloses a techniqueused to inspect defects by placing a plurality of reflecting mirrorsbetween a substrate to be inspected and a detection optical systemplaced above the substrate, and thereby acquiring images of scatteredlight from a plurality of directions. Moreover, JP-A-2008-261790 (PatentDocument 6) discloses a technique for extending a scattered-lightdetection range by cutting off two end portions of each of circularlenses and using these lenses as part of a detection optical system fordetecting scattered light from a plurality of directions. Besides,JP-A-2009-53132 (Patent Document 7) discloses a technique for inspectingdefects by conducting comparative processing of scattered-light imagesacquired from a plurality of directions.

If detectability of a detection optical system is enhanced in an attemptto detect finer defects, such changes as in ambient temperature and inatmospheric pressure will change imaging performance of the detectionoptical system, resulting in defect detection sensitivity decreasing.Techniques for improving this problem are described in, for example,JP-A-2002-90311, JP-A-2007-248086, and JP-A-2008-249571 (PatentDocuments 8, 9, and 10). The techniques disclosed in Patent Documents 8and 9 relate to correcting changes in imaging position due to changes intemperature and atmospheric pressure. The technique disclosed in PatentDocument 10 relates to controlling an internal temperature of aninspection device.

In connection with a scattered-laser-light detection type of defectinspection, “Principles of Optics” (M. Born, E. Wolf), CambridgeUniversity Press, pp. 774-785, (1999) (Non-Patent Document 1) introducesthe fact that intensity of a scattered-light signal from a microscopicobject whose diameter or radius is smaller than a wavelength of lightdecreases inversely with the sixth power of a size of the object andincreases in proportion to the fourth power of illumination wavelength.

In addition, the relational expression representing the relationshipbetween changes in ambient temperature and ambient air pressure and achange in the reflective index of air is shown in “The Reflective Indexof Air” (Bengt Edlen), Metrologia vol. 2, No. 2, pp. 71-80, (1966)(Non-Patent Document 2).

Furthermore, the relational expression representing the relationshipbetween a change in wavelength and a change in the reflective index of alens material is shown in “Zur Erklarung der abnormen Farbenfolge imSpectrum einiger Substanzen” (Wolfgang Sellmeier), Annalen der Physikand Chemie, pp. 272-282, (1871) (Non-Patent Document 3).

SUMMARY

As described earlier herein, in darkfield defect inspection of apatterned substrate, defect detection sensitivity depends greatly uponhow accurately a defect-diffracted/scattered detection beam of light, ora defect signal, can be detected in distinction frompattern-diffracted/scattered and underlayer-diffracted/scattereddetection beams of light, or background light noise. It has also beendescribed earlier herein that discrimination between a defect signal andbackground light noise is achievable by adopting any one of thetechniques utilizing the differences between the scattered beams oflight causing the defect signal and the background light noise, that is,the differences in the respective orientations of occurrence andpolarization states due to the differences in the shape, material, andother factors of the object causing the scattered light.

In darkfield inspection of a patterned substrate, on the other hand, adetection optical system is constituted by an imaging optical system, animage of the light scattered from the substrate to be inspected isacquired, and this acquired image undergoes processing for defectdetection. Accordingly, defect detection sensitivity is greatly dictatedby the quality of the scattered-light image acquired. For example, it isnecessary, in addition to detecting scattered light from a directiondifferent from the previous one and conducting optical filtering with aspatial filter, a polarizing filter, or the like, to construct theoptical system so that the image of the scattered light will have thequality needed to discriminate between a defect signal and backgroundlight noise.

As described earlier herein, to improve defect detection sensitivity, itis effective to increase the amount of information for defect detection,by detecting defects with a plurality of detection optical systems andacquiring, from one position of one object to be inspected, a pluralityof scattered-light images different in features and characteristics.During image processing, in particular, it is effective not only toprocess each of the scattered-light images independently, but also toconduct comparisons between the scattered-light images different infeatures and characteristics. In addition, realizing this requiresimproving the quality of the scattered-light images acquired by thedetection optical systems, and minimizing any differences in qualitybetween the scattered-light images acquired by the detection opticalsystems.

A challenge to be attained by the present invention is to realize theabove-described two requirements relating to the improvement of defectdetection sensitivity by comparative analysis of a plurality ofscattered-light images different in features and characteristics, thatis, (a) improving the quality of scattered-light images acquired by aplurality of detection optical systems, and (b) minimizing thedifferences in quality between the scattered-light images acquired bythe detection optical systems.

The inventions described in Patent Documents 1 to 7 relate to techniquesfor improving defect detection sensitivity by using an appropriatedetection optical system according to the direction in which light isscattered, and the invention in Patent Document 1 only detects theamount of light scattered and does not presuppose image acquisition. Atleast the invention in Patent Document 1 is therefore considered to beunable to meet the challenge that the technique of the present inventionis to attain.

The inventions described in Patent Documents 2 and 3 do not presupposeconducting comparisons between a plurality of scattered-light imagesdifferent in features and characteristics, and are thus considered to beunable to meet the challenge that the technique of the present inventionis to attain.

In the inventions according to Patent Documents 4 and 5, when aplurality of kinds of scattered-light images different in detectiondirection are acquired, the detection optical system to be used ischanged in configuration, which causes differences in quality betweenthe plurality of kinds of scattered-light images. For this reason, theinventions described in Patent Documents 4, 5 are considered to beunable to meet the challenge that the technique of the present inventionis to attain.

In the invention according to Patent Document 6 is disclosed a techniquethat relates to avoiding mutual interference of light between aplurality of detection optical systems arranged in different directions,and the technique uses a plurality of circular lenses whose portionsthat are likely to cause the mutual interference of light are cut off.The particular technique, however, does not envisage ensuring thequality of the scattered-light images obtained when the substrate to beinspected is detected from an oblique direction, so the technique isconsidered to be unable to meet the challenge that the technique of thepresent invention is to attain.

The invention described in Patent Document 7 relates to a method ofdetecting defects by arranging a plurality of detection optical systemsin different directions, detecting images different in scatteringdirection, and comparing the images, or a method of detecting defects bydetecting scattered light with a detection optical system having a largeNA value of at least 0.7, then branching an optical path and detectingimages different in scattering direction, and comparing the images. Whena plurality of detection optical systems are arranged in differentdirections, however, contention for a mounting space (i.e., the possibleinterference of light) between the plurality of detection opticalsystems will usually make it difficult to ensure a large detectionaperture. In the detection optical system with the large NA value of atleast 0.7, it is also difficult, in terms of lens design, to ensure along working distance (W.D.) between the lens end and the object to beinspected. This means that in the darkfield optical type of defectinspection, in particular, it is difficult to realize a configurationneeded to ensure the space for guiding laser illumination light (seeFIG. 4) to the wafer surface. In addition, the above invention does notenvisage ensuring the quality of the scattered-light images obtainedwhen the substrate to be inspected is detected from an obliquedirection, so the technique is considered to be unable to meet thechallenge that the technique of the present invention is to attain.

The inventions described in Patent Documents 8 to 10 relate totechniques for accommodating changes in imaging characteristics due toambient environmental changes, and the inventions in Patent Documents 8,9 only conduct corrections for in-focus position variations due to theambient environmental changes, and do not allow for other changes incharacteristics, so the corresponding techniques are considered to beunable to meet the challenge that the technique of the present inventionis to attain. The invention described in Patent Document 10 only holds aconstant temperature environment and does not allow for changes inatmospheric pressure, so the invention is considered to be unable tomeet the challenge that the technique of the present invention is toattain.

The present invention contemplates the improvement of defect detectionsensitivity, based on comparative analysis of a plurality ofscattered-light images different in features and characteristics, and anobject of the invention is to provide a defect inspection method anddefect inspection device that enables the improvement of quality ofscattered-light images acquired by a plurality of detection opticalsystems, and the minimization of any differences in quality between thescattered-light images acquired by the detection optical systems.

The present invention includes a plurality of means to solve theproblems. Among these means is a defect inspection method, whichincludes: an illumination step of irradiating a surface-patterned objectto be inspected, with light from an illumination optical system in sucha way as to form a linear illumination region on the surface of theobject; a detection step of converging via a detection optical systemthe light reflected/scattered from the object, then forming an opticalimage of the object surface on an image sensor, and converting thereflected/scattered light into an electrical signal; a defectdiscrimination step of extracting a defect signal by processing theelectrical signal that has been obtained by the photo-electricconversion; and a scanning step of moving the object in a mountedcondition and applying the detection step to the entire surface of theobject. The detection step is conducted using a plurality of detectionoptical systems and image sensors, and the defect discrimination step isconducted to extract the defect signal by comparing detection imagesobtained by the plurality of detection optical systems and imagesensors. The detection optical systems used in the detection step havethe same construction and are arranged so that respective optical axesform different angles of elevation in one plane perpendicular to asurface formed and defined by a longitudinal direction of the linearillumination region and a line normal to the object surface; detectionlenses used in the detection optical systems are composite lensassemblies, part of which include oval shaped lenses of a left-rightsymmetrical shape created by cutting off left and right end portions ofa circular lens rectilinearly, and rectilinear portions of the detectionlenses are disposed to be perpendicular to a surface formed by theoptical axes of the detection optical systems; in the illumination step,the longitudinal direction of the linear illumination region is formedto be perpendicular to the optical axes of the detection opticalsystems; and in the scanning step, scanning is conducted in a directionperpendicular to the longitudinal direction of the linear illuminationregion.

Another example that the present invention provides as the means forsolving the problems is a defect inspection method, which includes:irradiating a linear region on a surface-patterned sample mounted on atable which moves in a plane, with illumination light from an inclineddirection relative to a direction of a line normal to the sample;detecting from each of a plurality of directions an image of scatteredlight originating from the sample irradiated with the illuminationlight; and detecting a defect on the sample by processing signalsobtained by the detection of the images of the scattered light. The stepof detecting the scattered light image in the plural directions isperformed through oval shaped lenses in which elevation angles of theoptical axes thereof are different from each other, within one planeperpendicular to a plane formed by the normal to the surface of thetable on which to mount the sample and the longitudinal direction of thelinear region irradiated with the irradiation light, the oval shapedlenses being formed of circular lenses having left and right portionsthereof cut.

Yet another example that the present invention provides as the means forsolving the problems is a defect inspection device, which includes: atable unit adapted to move in a plane with a surface-patterned samplemounted on the table unit; an illumination optics unit that irradiates alinear region on the sample mounted on the table unit, with illuminationlight from an inclined direction relative to a direction of a linenormal to the patterned surface of the sample; a detection optics unitthat detects an image of scattered light originating from the sampleirradiated with the illumination light by the illumination optics unit;and an image-processing unit that detects a defect on the sample byprocessing a signal obtained from the image of the scattered light thatthe detection optics unit has detected. The detection optics unitincludes a plurality of detection optical systems arranged so that ovalshaped lenses in which elevation angles of the optical axes thereof aredifferent from each other are arranged within one plane perpendicular toa plane formed by the normal to the surface of the table unit on whichto mount the sample and the longitudinal direction of the linear regionirradiated with the irradiation light by the illumination optics unit.The detection optical systems each include an objective lens that is theoval shaped lenses formed of circular lenses having left and rightportions thereof cut.

In accordance with the present invention, adoption of the configurationoutlined above enables high-NA (numerical aperture) detection of imagesfrom a plurality of directions, and hence, realization of highlysensitive inspection by effective detection of the light scattered froma microscopic defect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a basic configuration of a defectinspection device in a first embodiment of the present invention;

FIG. 2 is a partial sectional view of a patterned substrate to beinspected, the sectional view showing a vertical structure of thepatterned substrate and the kinds of defects to be detected;

FIG. 3 is a diagram showing a flow of the steps of forming variouslayers of a semiconductor device, the flow diagram also listing names ofmaterials and typical defects on a layer-by-layer basis;

FIG. 4 is a plan view (a), side view (b), and front view (c) of an ovalshaped lens used in the first embodiment of the present invention;

FIG. 5 is a plan view (a) and front view (b) of oval shaped lenses asarranged in the first embodiment of the present invention;

FIG. 6 is a front view of oval shaped lenses that illustrates an examplein which the oval shaped lenses in the first embodiment of the presentinvention are constructed using composite lenses;

FIG. 7 is a plan view (a) showing an example in which various objectivelenses of detection units are each composed of a circular lens, and aplan view (b) showing an example in which the objective lenses are eachcomposed of an oval shaped lens;

FIG. 8 is a block diagram of a detection optical system that illustratesthin-line illumination as used in the first embodiment of the presentinvention;

FIG. 9A is a graph representing a relationship between atmosphericpressure and lens aberration;

FIG. 9B is a graph representing a relationship between lens aberrationand Strehl ratio;

FIG. 10 is a front view showing schematically a detection optical systemconfiguration including a mechanism to correct a change in ambient airpressure in a second embodiment of the present invention;

FIG. 11 is a perspective view of a patterned substrate as mounted on astage unit prior to defect inspection, the perspective view illustratinga direction of illumination by an illumination optical system in thesecond embodiment of the present invention;

FIG. 12 is a front view (a) representing a relationship between thepatterned substrate to be inspected and an objective lens of a detectionunit, and a plan view (b) representing a relationship between thepatterned substrate that has been illuminated with a thin linear beam oflight, and the objective lens of the detection unit;

FIG. 13 is a graph of changes in refractive index of air, plottedagainst changes in atmospheric pressure;

FIG. 14 is a graph of changes in refractive index of synthetic quartz,plotted against changes in wavelength of light passed through a lens;

FIG. 15 shows a first example of a mechanism which changes a wavelengthof the illumination light source; and

FIG. 16 shows a second example of a mechanism which changes thewavelength of the illumination light source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereunder, embodiments of the present invention will be described usingthe accompanying drawings.

While the following description takes defect inspection of asemiconductor wafer as an example, the present invention is not limitedto the example and can be applied to a method and device for inspectingother objects on which patterns are formed. For example, the inventioncan also be applied to inspection of substrates used for flat-paneldisplays such as a liquid-crystal display, plasma display, and organicEL display, and for patterned data-storage products such as discretetrack media (DTM) and bit-patterned media (BPM).

First Embodiment

FIG. 1 shows an example of a defect inspection device configuration as afirst embodiment. The defect inspection device according to the presentembodiment includes an illumination optics unit 10, a detection opticsunit 11, a data-processing unit 12, a stage unit 13, and a total controlunit 01.

The illumination optics unit 10 includes a light source 101, apolarization state controller 102, a beam-forming unit 103, and athin-line converging optics system 104. Illumination light that hasemitted from the light source 101 in this configuration is passedthrough the polarization state controller 102 and the beam-forming unit103, and then introduced into the thin-line converging optics system104. The polarization state controller 102 is a member including suchpolarizers as a half-wave plate and a quarter-wave plate. Thepolarization state controller 102, further fitted with a driving element(not shown) that is adapted to rotate about an optical axis of theillumination optical system, controls a polarization state of the lightused to illuminate a wafer 001 mounted on the stage unit 13. Thebeam-forming unit 103 is an optical unit that forms thin-lineillumination described later herein, and the optical unit includes abeam expander, an anamorphic prism, and the like.

The thin-line converging optics system 104, which includes a cylindricallens as its major element, illuminates a thin linear illumination region1000 of the wafer (substrate) 001 with illumination light formed into ashape of a thin line. The description of the present embodiment assumesthat as shown in FIG. 1, a cross direction of the thin-line illumination(i.e., a direction perpendicular to a longitudinal direction of thethin-line illumination region) is a stage scan direction 1300(x-direction) and the longitudinal direction of the thin-lineillumination is a y-direction.

In addition, in the present embodiment a narrow region is illuminated inthis way by the thin-line illumination, one of purposes of which is toimprove inspection throughput by enhancing intensity of the illumination(energy density of the illumination) for the object, that is, the wafer(substrate). To this end, the light source 101 is desirably a laserlight source, or a highly converging and highly coherent light sourcethat emits linearly polarized light. Additionally, as discussed in the“Background” hereof, reduction in a wavelength of the light source iseffective partly for increasing the amount of light scattered from adefect, and the present embodiment envisages an ultraviolet (UV) laseras the light source 101. For example, the embodiment uses either a YAG(Yttrium Aluminum Garnet)-THG (third-harmonic generation) solid-statelaser of 355 nm in wavelength, a YAG-FHG (fourth-harmonic generation)solid-state laser of 266 nm in wavelength, or a 213-nm, 199-nm, 193-nmsolid-state laser of a sum-frequency generation type based on acombination of YAG-FHG and YAG fundamental waves.

The light diffracted/scattered from the wafer 001 which has undergonethe thin-line illumination from the illumination optics unit 10 isdetected through the detection optics system 11. The detection opticssystem 11 includes three detection units, namely, 11 a, 11 b, and 11 c.Although the configuration with the three detection units is shown inthe present embodiment, the detection optics system is not limited tothe example and may use two detection units or at least four detectionunits. Hereinafter, for ease of distinction, constituent elements of thedetection unit 11 a as a first detection unit, those of the detectionunit 11 b as a second detection unit, and those of the detection unit 11c as a third detection unit, are expressed with suffixes “a”, “b”, and“c”, respectively, at ends of reference numbers.

The first detection unit 11 a includes an objective lens 111 a, aspatial filter 112 a, a polarizing filter 113 a, an imaging lens 114 a,and an image sensor 115 a. The second detection unit 11 b and the thirddetection unit 11 c also include substantially the same optical elementsas the above.

Operation of the first detection unit 11 a is described below. Thediffracted/scattered light from the wafer 001 is converged by theobjective lens 111 a, and an image of the wafer-scattered light isformed on the image sensor 115 a by the imaging lens 114 a. The seconddetection unit 11 b and the third detection unit 11 c also operate insubstantially the same form as the above. That is to say, thediffracted/scattered light is converged by objective lenses 111 b, 111c, and images of the wafer-scattered light are formed on image sensors115 b, 115 c by imaging lenses 114 b, 114 c. The objective lenses 111 a,111 b, 111 c here are each formed by, as shown in FIG. 1, a lensobtained by cutting off left and right end portions of a circular lensrectilinearly (hereinafter referred to as an oval shaped lens).Configurations and advantageous effects of the objective lenses will bedescribed in detail later herein.

The spatial filters 112 a, 112 b, 112 c in the detection optics system11 block the light regularly diffracted from cyclic patterns regularlyformed on the substrate, thereby reduce detection-associated backgroundnoise, and improve defect detection sensitivity. The polarizing filters(polarizers) 113 a, 113 b, 113 c are used to filter out a specificpolarization component included in detected light, thus reducebackground noise, and improve defect detection sensitivity.

The image sensors 115 a, 115 b, 115 c each convert a detected opticalimage into an electrical signal by photo-electric conversion. Ingeneral, charge-coupled device (CCD) sensors, complementary metal-oxidesemiconductor (CMOS) sensors, time delay integration (TDI) sensors, orother array sensors are used as the image sensors. Photodiode (PD)arrays, avalanche photodiode (APD) arrays, or photomultiplier tube (PMT)arrays may be used as alternatives to the image sensors.

The above-mentioned thin linear illumination region 1000 on thesubstrate is illuminated so as to scatter the illumination lightcollectively toward a detection range of the image sensor 115 forenhanced illumination efficiency (this avoids inefficient illuminationthat causes scattering in a direction overstepping the detection rangeof the image sensor). The present embodiment assumes that the imagesensors 115 a, 115 b, 115 c are line sensors.

The three detection units, 11 a, 11 b, 11 c, arranged in the detectionoptics system 11 of the present embodiment are of the same construction.This reduces any differences in quality between the scattered-lightimages detected by the detection units, and thereby raises extractionaccuracy of a defect signal, based on comparison results of thescattered-light images obtained by the different detection units duringimaging. The arrangement of the detection units having the sameconstruction also helps reduce a manufacturing man-hour requirement andmanufacturing cost of the inspection device.

The data-processing unit 12 uses signal-processing units 121 a, 121 b,121 c to conduct A-D conversion of the detection image signals which thefirst, second, and third image sensors, 115 a, 115 b, 115 c, haveacquired by photo-electric conversion, and then to generate inspectionimage data by conducting a process such as noise reduction or noiseremoval.

Reference number 122 denotes an image-processing unit, in which theinspection image data that the signal-processing units 121 a, 121 b, 121c have generated undergoes image processing based on comparison withreference image data and a defect signal is extracted from comparisonresults. The reference image data may be created from design datarelating to the circuit patterns formed on the wafer, or may be storedimage data obtained after imaging of those patterns on differentsections of the wafer that originally have the same shape as that of anyone of the patterns, or may be image data obtained between differentdetection units by imaging the same section on the wafer.

In the extraction of the defect signal, information contained in thescattered-light images which have been obtained by the first, second,and third detection systems (optics) undergoes processing and the defectis extracted. During the extraction of the defect signal, not only theimage obtained by each detection system is subjected to independentprocessing, the images obtained by the different detection systems arealso subjected to comparative processing.

The control unit 01 controls the illumination optics system 10, thedetection optics system 11, the data-processing unit 12, and the stageunit 13.

The stage unit 13, which is a mechanism that moves the mounted wafer 001in xyz directions, includes an X-stage 131 and a Y-stage 132, each ofwhich has an x-axial or y-axial movement stroke to enable the detectionoptics system 11 to inspect the entire wafer surface, and a Z-stage 133has a z-axial movement stroke to control a z-position of the wafersurface (the surface to be inspected) within a focus range of thedetection optics system 11, even if the wafer is not uniform inthickness.

Stage movements of the stage unit 13 during inspection are controlled inthe following fashion. As shown in FIG. 1, the wafer 001 is illuminatedat the thin linear illumination region 1000 having a longitudinaldimension as length “Li” in a direction of the y-axis. Duringinspection, the X-stage 131 continuously moves the wafer 001 in adirection 1300 of the x-axis and the three detection units of thedetection optics system 11 scan the wafer to acquire images.

In the example of FIG. 1, where, for example, a left edge of the waferis taken as a starting position of movement, the X-stage 131 moves thewafer until the stage has reached an opposite edge (right edge) of thewafer, and the detection optics system 11 scans across the wafersurface, that is, between the left and right edges. After the opposite(right) edge of the wafer has been reached, in order to provide for nextscan, the Y-stage 132 moves the wafer in steps through the length “Li”of the illumination region 1000 in the direction of the y-axis, then theX-stage 131 continuously moves in a direction opposite to that of theprevious movement, and the detection optics system 11 acquires waferimages by scanning the wafer in substantially the same manner as that ofthe previous scan. The entire wafer surface is inspected by repetitionof such processing.

During scanning, if the wafer goes out of the focus range of thedetection optics system 11, the quality of the wafer images acquiredwill deteriorate and defect detection sensitivity will decrease. Inorder to avoid this, the z-position of the wafer surface is controlledby the Z-stage 133 to always stay within the focus range of thedetection optics systems 11 during scanning. The z-position of the wafersurface is detected by a wafer surface z-position detection device notshown.

Defocusing significantly affects the quality of the acquired waferimages and can be a cause of a significant decrease in defect detectionsensitivity. In order to avoid this, the illumination optical system andthe detection optical system are constructed as follows in the presentembodiment: the detection units having the same construction in thedetection optics system are arranged so that respective optical axesdiffer from one another in detection angle of elevation in one plane(hereinafter, this plane is referred to as the detection optical-axisplane) and so that the detection optical-axis plane is perpendicular toa plane formed by two elements, that is, a line normal to the objectsurface to be inspected, and the longitudinal direction of the thinlinear illumination region 1000.

Since the detection units are arranged in this form, when the samedetection optics system is disposed in plurality and scattered light isdetected from different directions, distances from those points within adetection range, on the surface to be inspected, that are detected bythe image sensors (line sensors) of the detection optics system 11, todetection surfaces of the image sensors, can be kept the same and evenwithout a special mechanism, scattered-light images in focus can beobtained over entire detection regions of the image sensors (linesensors).

The objective lenses 111 a, 111 b, 111 c of the present embodiment,described earlier, are each formed using an oval shaped lens of aleft-right symmetrical shape that is obtained by cutting off the leftand right end portions of a circular lens rectilinearly, and arearranged so that the cut rectilinear portions are perpendicular to thedetection optical-axis plane described above. When a plurality ofdetection units are arranged, therefore, the use of the oval shapedlenses, compared with the use of ordinary circular lenses, enables theextension of a detection aperture for enhanced capturing efficiency ofthe scattered light. The use of the oval shaped lenses also enables theacquisition of in-focus scattered-light images over the entire detectionregions of the image sensors (line sensors) 115 a, 115 b, 115 c. The useof the oval shaped lenses additionally enables the detection of uniformimage quality over the entire detection regions of the image sensors(line sensors) by constructing symmetrical optics with respect to theplane formed by the longitudinal direction of the image sensors (linesensors) 115 a, 115 b, 115 c and the optical axes of the detection units11 a, 11 b, 11 c.

In addition, the plurality of detection units (in the presentembodiment, three units, namely 11 a, 11 b, 11 c) are arranged so thatthe respective optical axes are symmetrical with respect to the planeformed by two elements, that is, the line normal to the object surfaceto be inspected, and the longitudinal direction of the thin linearillumination region 1000 on the object surface. When the images acquiredby different sets of detection optics systems undergo comparativeprocessing for the extraction of a defect signal, the above arrangementof the detection units facilitates comparative processing of thoseimages. For example, detecting one position from the left and rightsides thereof at the same detection angle of elevation in the abovearrangement enables the acquisition of two scattered-light images havingsubstantially the same quality and reflecting only the difference in thedirection of occurrence of the scattered light, and then executingcomparative processing of the two images enables highly accurateextraction of a defect signal. Furthermore, at least one of theplurality of detection units (in the present embodiment, three units,namely 11 a, 11 b, 11 c) in the present embodiment is disposed so thatthe corresponding optical axis is in alignment with the line normal tothe object surface to be inspected. This disposition facilitates devicestate monitoring with reference image quality assigned to the imageacquired through the particular detection unit.

As will be described later herein, the detection optical systems foroblique detection (in the present embodiment, the detection units 11 b,11 c) are liable to significantly deteriorate the quality of thedetection images (scattered-light images) in case of defocusing. In thepresent embodiment, a plurality of detection units having the sameconfiguration are arranged and if their original performance isexhibited, the scattered-light images acquired by the detection unitswill have substantially equal quality. At this time, however, if alldetection units of the device are constructed only of theoblique-detection optical systems, it is estimated that all the unitswill cause similar image deterioration due to defocused obliquedetection. In addition, if this actually occurs, it will be difficult todetermine to what extent the actually acquired image quality satisfiesthe image quality that originally ought to be obtained.

If one detection unit (in the configuration of FIG. 1, the detectionunit 11 a) is disposed so that its optical axis is in alignment with theline normal to the object surface to be inspected, the deterioration ofa detection image in the corresponding optics due to defocusing will beless significant than that of the other detection units for obliquedetection (in the present embodiment, the detection units 11 b and 11c). Accordingly, when the quality of the scattered-light image acquiredby the particular optics is adopted as a reference, the quality of thescattered-light images acquired by the other units can be appropriatelyevaluated and device state monitoring and on-trouble adjustment becomeeasy.

A configuration of the oval shaped lenses in the present embodiment isdescribed below using FIGS. 4 to 7.

FIG. 4 is an explanatory diagram illustrating a single-lens shape of oneoval shaped lens, 111. This planar shape of the oval shaped lens 111 iscreated by, as shown in FIG. 4( a), cutting a circular lens along tworectilinear cutting planes, 1110, thereof into an oval shaped of aleft-right symmetrical shape. A side shape is created by, as shown inFIG. 4( c), cutting the lens obliquely so that if a detection apertureangle obtained in a short-side direction of the single lens by combiningthe lens in plurality to construct a composite lens assembly isexpressed as θW2, a distance from a focus plane of each lens as L, andhalf width of the lens as W2, then a relationship of W2≈L·tan θW2exists. Thus the x-axial detection aperture θW2 of the lens, shown inFIG. 4( c), differs from a y-axial aperture θW1 shown in FIG. 4( b), andit follows that θW1>θW2. The following describes how the oval shapedlenses are to be arranged in the actual device to realize such arelationship.

FIG. 5 is a diagram that illustrates layout of the oval shaped lenses inthe inspection device. Section (a) of FIG. 5 is a plan view of the ovalshaped lenses, and section (b) is a front view thereof. The three ovalshaped objective lenses, 111 a, 111 b, 111 c, in an x-y plane of FIG.5(a) all have the same aperture, but since the optical axes of theobjective lenses 111 b, 111 c are inclined and these two objectivelenses are shown in x-y plane view, the two objective lenses aredepicted as if both were seemingly smaller than the objective lens 111a.

The three oval shaped objective lenses, 111 a, 111 b, 111 c, arearranged so that respective focus positions match the position of thethin linear illumination region 1000. At this time, the optical axes ofthe oval shaped objective lenses 111 a, 111 b, 111 c meet together onone such planar section of the detection optical-axis plane 1112 that isperpendicular to the plane formed by two elements, that is, the line1111 normal to the surface of the wafer 001, and the longitudinaldirection (y-axis direction) of the thin linear illumination region1000. In addition, the optical axes are of symmetrical layout about theline 1111 normal to the surface of the wafer 001. The cutting planes1110 a, 1110 b, 1110 c of each lens are as close as possible to oneanother, and are also substantially parallel to one another.Furthermore, the cutting planes 1110 a, 1110 b, 1110 c of the lens areoriented in a direction parallel to the longitudinal direction of thethin linear illumination region 1000, and when inspection images areacquired, the wafer is scanned in a direction 1300 perpendicular to thedirection of the cutting planes.

The detection aperture of the lens has the angle of θW2 in thex-direction and the angle of θW1 in the y-direction. While the aperturesize of the lens as considered as an independent element is greater inthe y-direction than in the x-direction, combination of the lenses 111a, 111 b, 111 c enables an aperture of the entire composite lensassembly to be extended in the x-direction.

FIG. 6, which assumes that the actual objective lenses are compositelens assemblies each formed from a combination of single lenses, is anexplanatory diagram illustrating an example in which each of thecomposite lens assemblies is constituted by oval shaped lenses. FIG. 6shows an example in which each objective lens assembly 111 a, 111 b, 111c is constructed using 12 composite lenses. In this case, not all of the12 lenses need to be oval shape lenses. Since an increase in distancefrom the wafer 001 also causes an increase in distance between theoptical axes of the lenses, if the lenses are circular, interferencebetween them is likely, so these interfering lenses are preferablyreplaced by oval shaped lenses.

In the present embodiment, since interference between circular lenses islikely, nine lenses closer to the wafer are oval shaped lenses. A basicstate of cutting is the same as that described in FIG. 4. That is tosay, the nine front lenses of each of the objective lens assemblies 111a, 111 b, 111 c are formed from circular lenses cut for a detectionaperture angle θW, along cutting planes 1110 a, 1110 b, 1110 c.

The three rear lenses, which do not interfere with each other, need nocutting, so they are not cut. In addition, as in FIG. 5, the threeobjective lenses, 111 a, 111 b, 111 c, are arranged so that respectivefocus positions match the position of the thin linear illuminationregion 1000. At this time, the optical axes of the oval shaped objectivelenses 111 a, 111 b, 111 c meet together on one plane (equivalent to thedetection optical-axis plane 1112) that is perpendicular to the planeformed by two elements, that is, the line 1111 normal to the surface ofthe wafer 001, and the longitudinal direction (y-axis direction, notshown) of the thin linear illumination region 1000. In addition, theoptical axes are of symmetrical layout about the line 1111 normal to thesurface of the wafer 001. The cutting planes 1110 a, 1110 b, 1110 c ofeach lens are as close as possible to one another, and are alsosubstantially parallel to one another.

FIG. 7 is an explanatory diagram that illustrates advantages arisingfrom adopting oval shaped lenses. FIG. 7( a) shows apertures obtainedwhen light is detected from three different directions using circularlenses 111 na, 111 nb, 111 nc of the same size. The apertures of thelenses are all of the same size and circular, but since the optical axesof the objective lenses 111 nb, 111 nc are inclined and these twoobjective lenses are shown in x-y plane view, the two objective lensesare depicted as if both were seemingly smaller than the objective lens111 na.

To avoid lens-to-lens interference in this case, it is necessary to makethe lens apertures, and since these apertures are circular, it isfurther necessary to make the apertures smaller in both an x-directionand a y-direction. The detection optics system in the present exampleassumes forming wafer images with the imaging optics, and for thispurpose, envisages a condition of arranging the plurality of objectivelenses so that the respective optical axes meet together in one plane.For this reason, if a plurality of circular lenses are arranged on theabove assumption, this arrangement is likely to cause an inconvenienceof the detection aperture sizes being very much limited, especially they-axial dimension of each detection aperture becoming too small.

On the other hand, if as shown in FIG. 7( b), oval shaped objectivelenses 111 a, 111 b, 111 c are adopted and x-axial and y-axial aperturedimensions of each of the objective lenses are made to be optionallysettable, it suffices just to make the x-axial aperture of one objectivelens smaller for avoidance of interference and arrange a correspondinglylarger number of lenses in the x-direction. Additionally, any necessaryy-axial aperture dimensions of the lenses can be set, irrespective ofthe x-axial aperture dimension(s), and even if an image is to bedetected using a plurality of sets of detection optics systems,capturing efficiency of scattered light can be significantly improved incomparison with a case in which the detection optics system isconstituted by circular lenses.

Next, necessity for the thin-line illumination in the present embodimentis described below using FIG. 8. In addition to objective lenses 111 aand 111 b, a third objective lens is actually located to the left of theobjective lens 111 a, and thus three detection units constitute adetection optics system. For simplicity, however, the detection unitactually present at the left of the objective lens 111 a is omitted andan example assuming that the detection optics system is constituted bytwo detection units is described here.

This example assumes that a second detection unit shown with suffix “b”has an optical axis inclined at an elevation angle θd with respect tothe surface of the wafer 001 that is to be inspected, and that theobjective lens 111 b has an aperture angle θW, that is, the objectivelens 111 b has an x-axial numerical aperture NAx represented as follows:NAx=sin θW  (Numerical expression 1)

When the wavelength of the illumination light source is expressed as λ,if a depth of focus of the objective lens 111 b is expressed as DOF,then:DOF=λ/(sin θW)²  (Numerical expression 2)

The thin linear illumination region 1000 on the wafer is illuminatedwith an illumination width “Wi” of light. If the illumination lightoversteps a DOF range of the objective lens 111 b of the seconddetection unit, scattered light from regions outside the DOF range willenter and images of the scattered light will contain a blurringcomponent, which will in turn deteriorate the image quality of thescattered light, thus reducing defect detection sensitivity. To preventthe reduction in sensitivity from occurring, it is necessary that thethin linear illumination region 1000 and the illumination width “Wi”should fall within the DOF range of the objective lens 111 b of thesecond detection unit, that is, that the following relationship shouldhold:Wi<DOF/sin θd  (Numerical expression 3)

In addition to this, depending on control accuracy of the z-stage, thedetection position of the wafer is likely to move in the direction ofthe optical axis of the second detection optical unit. If the controlaccuracy of the z-stage is taken as ±Δz, the change in the detectionposition can be expressed as follows:±Δz/cos θd  (Numerical expression 4)

Putting these together, the following becomes the condition necessary toacquire blur-free images of scattered light in the oblique-detectionoptical systems:DOF/sin θd>Wi+2×(Δz/cos θd)  (Numerical expression 5)

When a magnification of the second detection optical unit is expressedas M, the image sensor (line sensor) 115 b in the oblique-detectionoptical system 12 desirably has the following value as a pixel size Wd1in the scanning direction of the x-stage:Wd1≧M×Wi×sin θd  (Numerical expression 6)

This is because the image sensor 115 b needs to detect the scatteredlight originating from all illumination regions, improve detectionefficiency of the scattered light, and hence improve inspectionthroughput. In other words, if the pixel size Wd1 of the image sensor115 b is such thatWd1<M×Wi×sin θd  (Numerical expression 7)

and the detection range is limited to a portion of the illuminationrange, then the illumination light falling outside the detection rangeof the image sensor 115 b will not be used effectively, the amount oflight detected will decrease, and inspection throughput will alsodecrease.

Similarly, the image sensor 115 a of a first detection unit shown withsuffix “a” desirably satisfies the following relationship in terms ofillumination light utilization efficiency:Wd1≧M×Wi  (Numerical expression 8)

For reduced device costs, the inspection device of the presentembodiment assumes that the respective objective lenses 111 a, 111 b,imaging lenses 114 a, 114 b, and image sensors 115 a, 115 b of the firstand second optical units for detection are common in specifications.Depending on the device configuration, therefore, the larger of thevalues predetermined per numerical expressions 6 and 8 can be set as apixel size Ws1 of the image sensors 115 a, 115 b in the scanningdirection of the stage.

A pixel size Wd2 of the image sensors 115 a, 115 b, in a direction(y-direction, sensor arrayal direction) perpendicular to the scanningdirection of the stage, does not need to be the same as Wd1. Signals aredesirably sampled at a rate N (N=1, 2 . . . ) based on y-axialresolution of the objective lenses 111 a, 111 b, that is, on numericalexpression 9 defined from the formula relating to the Rayleigh'sdiffraction limit. Briefly, a preferable value of the pixel size is:Wd2=(0.61×λ/NAy)/N (N=1,2 . . . )  (Numerical expression 9)

An appropriate sampling rate N to be assigned subject to the Nyquisttheorem is at least 2, and if possible, nearly 4. However, even if alarger value is assigned (i.e., even if the pixel size is made smallerthan necessary), this is ineffective in terms of the improvement ofinspection image quality and only results in narrowed inspection areaand hence in reduced inspection throughput, so that the appropriatevalue within the above range needs to be set.

For these reasons, the pixels of the image sensors in the presentembodiment are desirably the rectangular pixels that satisfy Wd1>Wd2,that is, the pixels whose size generally differs between the scanningdirection of the stage and the direction perpendicular to this scanningdirection.

In the present embodiment, which envisages the use of the oval shapedlenses, whereas the x-axial lens numerical aperture NAx is restricted bythe arrangement of the lenses, the y-axial lens numerical aperture NAyis not subject to the restriction. Increasing NAy without any suchrestriction, therefore, enables Y-axial resolution to be raised and thusthe image quality of the scattered light to be correspondingly enhanced.X-axial resolution can likewise be raised, regardless of the aperturesizes of the lenses, by reducing the line width “Wi” of the thin-lineillumination below the x-axial lens resolution of 0.61×λ/NAx andnarrowing the illumination range. The use of the oval shaped lenses isparticularly effective in a case that the number of detection units isincreased and the x-axial aperture sizes of the objective lenses arecorrespondingly reduced.

An example in which the three detection units, 11 a to 11 c, of thedetection optics system 11 all include the same optical elements hasbeen described in the above embodiment. However, the present inventionis not limited to this configuration and may adopt a configuration inwhich the objective lens 111 a of the first detection unit 11 a is madelarger than the objective lenses 111 b and 111 c of the second and thirddetection units 11 b and 11 c. Thus, the light scattered perpendicularlyrelative to the wafer 001 and the light scattered in a vicinity thereofwill be converged in greater amounts by that objective lens 111 a toform the images obtained. With this configuration, the detection opticssystem can have the NA of the first detection unit 11 a increased anddetect even more microscopic defects with the first detection unit 11 a.

Second Embodiment

Changes in ambient environment significantly affect the image quality ofthe scattered light. Although changes in temperature can be accommodatedby merely providing a temperature control mechanism inside the device,it is difficult in terms of costs to provide, against changes inatmospheric pressure, a structure or mechanism that keeps an internalatmospheric pressure of the entire device constant.

FIGS. 9A and 9B are diagrams that explain impacts of atmosphericpressure changes upon the deterioration of image quality. FIG. 9A showscalculation results that indicate how changes in atmospheric pressurevary lens aberration. For example, even if a lens is assembled andadjusted under an environment of 1,000 hPa and aberration is controlledbelow 0.1λ, a decrease of the atmospheric pressure to 850 hPa degradesaberration to 0.2λ.

The degradation in aberration is a component that cannot be sufficientlycorrected by adjustment of the imaging position, as in the prior-artdevices discussed earlier herein. FIG. 9B illustrates the adverseeffects of such degradation upon images of scattered light. In the casethat aberration degrades from 0.1λ to 0.2λ, the Strehl ratio thatrepresents theoretical point-imaging performance decreases below ⅓ ofits original value. This means that a blur of the image degrades by afactor of three. If such image deterioration occurs, defect detectionperformance that has been obtained under the original environment of1,000 hPa in atmospheric pressure cannot likewise be achieved under theenvironment of 850 hPa in atmospheric pressure.

In the present embodiment, therefore, a function that prevents imagequality from deteriorating even if a change in atmospheric pressureoccurs during inspection is imparted to the defect inspection devicedescribed in the first embodiment.

FIGS. 10 and 11 are diagrams that explain mechanisms assigned to lensesto correct the deterioration of image quality. Not all these correctionmechanisms need to be provided and it suffices if, among thesemechanisms, at least one or more appropriate kinds of mechanisms areadded according to mechanism-mounting conditions and in a feasiblecorrection range that does not cause the deterioration of thescattered-light images detected.

The configuration shown in FIG. 10 relates to the optics correspondingto the first detection unit 11 a, second detection unit 11 b, and thirddetection unit 11 c of the first embodiment described using FIG. 1. Inthe present (second) embodiment, constituent elements not shown are thesame as those described in the first embodiment, and description ofthese elements is therefore omitted hereinafter.

Referring to FIG. 10, elements 1111 a, 1111 b, 1111 c for moving imagesensors in directions of optical axes are mechanisms that move the imagesensors in directions of arrows, that is, directions along optical axesof imaging lenses 114 a, 114 b, 114 c, in response to changes in imagingpositions of the lenses due to changes in ambient air pressure.Mechanisms 1112 a, 1112 b, 1112 c provide aberration correction byinserting/removing parallel flat plates. A change in performance of alens, associated with a change in ambient air pressure, is due to achange in the refractive index of air, caused by the change in ambientair pressure. In order to compensate for a decrease in the refractiveindex of air due to a decrease in atmospheric pressure, the parallelflat plates, which are high-refractive-index media, are insertedinto/removed from optical paths of the imaging lenses 114 a, 114 b, 114c, to correct aberration.

Lens actuators 11131 a, 11131 b, and 11131 c each move one lens 1113 a,1113 b, or 1113 c within lens groups constituting the imaging lenses 114a, 114 b, 114 c, in a direction of an arrow, that is, the directionalong the optical axis of the imaging lens 114 a, 114 b, 114 c.Controlling a position of the lens 1113 a, 1113 b, 1113 c via thecorresponding lens actuator 11131 a, 11131 b, and 11131 c enablesperformance of the imaging lens 114 a, 114 b, 114 c to be corrected andthus a change in the performance of the lens due to a change inatmospheric pressure to be compensated. Air pressure controllers 1114 a,1114 b, 1114 c control internal air pressures of lens tubes 1110 a, 1110b, 1110 c by keeping the inside of each lens tube 1110 a, 1110 b, 1110 cairtight, thereby to maintain constant lens performance. With the airpressure controllers, internal environments of the lens tubes 1110 a,1110 b, 1110 c can be blocked from their external environments tomaintain the internal air pressures of the lens tubes 1110 a, 1110 b,1110 c at the same level as during lens assembly and adjustment, andhence to maintain constant lens performance even under changing airpressures.

FIG. 10 shows an example in which the detection units are provided witha mechanism that corrects a change in lens performance due to a changein atmospheric pressure. This correction mechanism can also be providedin illumination optics systems. An example of illumination opticssystems provided with the correction mechanism is described below usingFIG. 11.

The reason why the change in lens performance due to a change inatmospheric pressure occurs is that the change in atmospheric pressurecauses a change in the refractive index of air, a medium that fills in aspace between lenses. When atmospheric pressure decreases, the densityof air also decreases, which in turn reduces the refractive index ofair.

A beam of light that passes through lenses is bent according toparticular differences between refractive indexes of the lens materials(glass, quartz, or others) and the refractive index of air, the mediumlying between the lenses. Thus, scattered light that has originated fromthe wafer propagates through the lenses and forms an image on an imagesensor. If the ambient air pressure changes and thus the refractiveindex of air changes, an extent to which the beam of light that passesthrough the lenses is bent will also change and a state of the imageformed on the image sensor will change as a result. Normal lens designis based on a prerequisite of 1 atmosphere (1,013 hPa) in ambient airpressure, and lenses are designed to develop best imaging performanceunder that environment. Imaging performance deteriorates for atmosphericpressure variations departing from the prerequisite.

The refractive indexes of the materials, on the other hand, differaccording to a wavelength of the light passing through them. Utilizingthis relationship allows a decrease in the imaging performance of thelenses due to a change in atmospheric pressure to be corrected byvarying the wavelength of the light passing through the lenses, that is,the wavelength of the illumination light source.

FIG. 13 is a graph that indicates changes in the refractive index ofair, plotted against changes in atmospheric pressure. According toNon-Patent Document 2, the relationship between the refractive index ofair, n_(air), air temperature T (° C.), and atmospheric pressure (Torr),is represented by numerical expression (10) as follows:n_(air)=1+(3.83639×10−7×P)(1+P(0.817−0.0133T)×10−6)/(1+0.03661T)  (Numericalexpression 10)

FIG. 13 is a graph based on numerical expression 10, representing howthe refractive index of air (−n_(air)) will vary as atmospheric pressurechanges from 850 to 1,020 [hPa]. The variation in the refractive indexof air (n_(air)) is nearly 4.56×10⁻⁵ for the change of 170 [hPa] inatmospheric pressure. As can be seen from this fact, when theatmospheric pressure changes, the refractive index also changes. Thesechanges in the refractive index of air (n_(air)) cause such changes inlens performance that are shown in FIGS. 9A, 9B.

FIG. 14, on the other hand, is a graph based on the Sellmeier's equation(Non-Patent Document 3), representing changes in refractive index(n_(siO2)) of synthetic quartz, a typical lens material, plotted againstchanges in the wavelength of the light passing through the lens. Whenthe wavelength increases, the refractive index decreases, and when thewavelength increases by 0.1 nm (100 μm) from 265.95 nm to 266.05 nm, therefractive index decreases by nearly 4.33×10⁻⁵.

Utilizing these relationships makes it possible, when ambient airpressure decreases (the refractive index of air, n_(air), decreases), tocontrol the wavelength of the light source for a shift to a greaterwavelength, and thus to keep refractive power of the lens constant, thatis, maintain a constant difference in refractive index between air andsynthetic quartz n_(si02)−n_(air)), by reducing the refractive index ofsynthetic quartz (n_(si02)) as well. Thus, even when the ambient airpressure changes, the performance of the lens can be kept substantiallyequal to its design value based on the prerequisite of 1atmosphere=1,013 hPa.

Next, mechanisms that change the wavelength of the illumination lightsource are described below using FIGS. 15 and 16.

As discussed earlier herein, a high-coherence and high-powershort-wavelength light source is desirable for the darkfield type ofdefect inspection device. Mechanisms based on this are also described inthe examples below.

FIG. 15 shows a first example of a mechanism which changes thewavelength of the illumination light source. Reference number 15Adenotes a seed laser section, 15B an amplifier section, and 15C awavelength converter section. Reference number 150 denotes a laser diode(LD), 1500 a passive fiber, 1501 a coupling, and 1502 an amplifyingfiber. Reference numbers 1503B1 and 1503B2 denote fiber Bragg gratings(FBGs). Reference number 154 denotes a nonlinear optical crystal.

Referring to the seed laser section 15A, laser light with a wavelengthλ1 is emitted from the LD 150 and introduced into the amplifier section15B via the passive fiber 1500.

Referring to the amplifier section 15B, the amplifying fiber 1502 is anoptical fiber doped with rare earthes, and the FBGs 1503B1, 1503B2placed across the amplifying fiber 1502 function as diffraction gratingsto generate periodic variations in the refractive index of the passivefiber 1500. Thus, only wavelengths that satisfy a Bragg reflectioncondition created by a period of the gratings are reflected, which formsan optical cavity, amplifies the incident λ1 laser light, then emits λ2laser light, and admits the λ2 laser light into the wavelength convertersection 15C.

Referring to the wavelength converter section 15C, the nonlinear opticalcrystal 154 includes a barium borate (BBO) crystal (βBaB204), a lithiumtriborate (LBO) crystal (LiB305), a KTP crystal (KTiOPO4), and a lithiumniobate crystal (LiNbO3). The nonlinear optical crystal 154 receives theincident λ2 laser light and emits high-harmonic λ3 laser light. Withthis configuration, high-power and short-wavelength laser light can beemitted.

In the other constituent elements in FIG. 15, 151A, 151B1, 151B2, 151Care temperature control units, and the temperature control elements152A, 152B1, 152B2, 152C are Peltier elements or heaters. Temperaturesensors 153A, 153B1, 153B2, 153C work to sense temperature states ofvarious constituent elements and send information to the correspondingtemperature control units 151A-151C, and the temperature control units151A-151C control the temperature control element 152A and correspondingtemperature control elements 152B, 152C to maintain the variousconstituent elements at required temperatures.

The LD 150, FBGs 1503, 1504, and nonlinear optical crystal 154 here havea characteristic in that each changes a corresponding wavelengthaccording to temperature. In the LD 150, for example, when the elementincreases in temperature, the wavelength of the laser light emitted willshift to a greater wavelength. This also occurs in the FBGs 1503,1504.When the FBG increases in temperature, thermal expansion spreads aspacing of its diffraction gratings and the Bragg wavelength shifts to agreater wavelength. In addition, an increase in a temperature of thenonlinear optical crystal 154 causes a change in its refractive index,thus shifting the wavelength of the higher-harmonic light.

These characteristics can be used to shift the wavelength of theillumination light source depicted in the overall block diagram of FIG.15, by controlling the temperatures of the constituent elements. Thisfeature, in turn, can be used to correct the lens characteristics for anenvironmental change.

FIG. 16 shows a second example of a mechanism which changes thewavelength of the illumination light source. Detailed description ofconstituent elements assigned the same reference numbers as in FIG. 15is omitted herein.

In the present example, the laser light of a λ1 wavelength that has beenemitted from the seed laser section 15A is admitted into a wavelengthconverter section 16B. The wavelength converter section 16B activatesthe nonlinear optical element 154 to form an optical cavity via mirrors161A, 161B, 161C, 161D, on the optical path. In this configuration, thenonlinear optical element 154 is temperature-controlled by thetemperature control unit 151C, the temperature control element 152C, andthe temperature sensor 153C, and the mirror 161C is moved by a mirroractuator mechanism 162. These actions change cavity length and thusallow wavelength shifting of the λ2-wavelength laser light which hasbeen emitted from the wavelength converter section 16B. This feature canbe further used to correct the lens characteristics for an environmentalchange.

Third Embodiment

A third embodiment relates to a direction of illumination. As describedearlier herein, the present embodiment presupposes that the longitudinaldirection of the thin linear illumination region 1000 is set to be they-axis direction, but this is not intended to limit the direction of theillumination.

Constituent elements of the present embodiment, such as the illuminationoptics unit 10 and the detection optics system 11, are substantially thesame as in the defect inspection device of the first embodimentdescribed using FIG. 1, and the present (third) embodiment differs fromthe first embodiment in terms of a location of the illumination opticsunit 10 relative to the detection optics system 11.

As shown in FIG. 11, a plane 1010, formed by a line 1111 normal to asurface of a wafer 001 and an optical axis of the illumination light,may have an arbitrary azimuth angle “φi” with respect to the y-axis.Thus, as described earlier herein, different scattered-light componentswill enter the detection optical units arranged to be left-rightsymmetrical with respect to the normal 1111 to the surface of the wafer001, and detection sensitivity will thus improve.

The section (a) of FIG. 12 is a front view representing a relationshipbetween the patterned substrate to be inspected and an objective lens ofthe detection optics system, and the section (b) of FIG. 12 is a planview representing a relationship between the patterned substrate thathas been illuminated with a thin linear beam of light, and the objectivelens of the detection optics system.

However, as shown in the section (a) of FIG. 12, when the objective lens111 a having the same optical axis as the normal 1111 to the surface ofthe wafer 001 is placed, it is effective to set “φi” in a limited rangeso that as shown in the section (b) of FIG. 12, diffracted light 1020from a circuit pattern 010 extending in parallel in an x-direction onthe wafer will enter the objective lens 111 a.

The reason for this is described below. In the present embodiment, thesecond and third detection units described in FIG. 1 but not shown inFIG. 12 are arranged to be left-right symmetrical with respect to thenormal 1111 to the surface of the wafer 001, and these detection unitsdetect substantially the same scattered-light images. Thescattered-light images that the detection units have thus acquiredreflect only a difference in the direction of the scattered light. Thepresent embodiment assumes highly sensitive detection of a defect bycomparing these images of the scattered light. If the diffracted light1020 from the circuit pattern 010 enters at least one of objectivelenses 111 b, 111 c of the second and third detection units, however, asignificant difference will occur between the two images of thescattered light, and this difference will make it impossible to comparethe scattered-light images that reflect only the difference in thedirection of the scattered light.

This, in turn, makes it necessary to conduct the illumination so that anillumination converging angle “θi”, the illumination azimuth angle “φi”with respect to the longitudinal direction (y-axis) of the linearillumination in the object plane, and a detection angle “θW” of thefirst detection optics system having the same optical axis as the normalto the object surface, with respect to a direction perpendicular to therectilinear portion of the lens, that is, in the section (b) of FIG. 12,the x-direction, will satisfy a relation of φi≦θW−θi.

REFERENCE SIGNS LIST

001 . . . wafer 01 . . . total control unit 10 . . . illumination opticsunit 101 . . . light source 102 . . . polarization state controller 103. . . beam-forming unit 104 . . . thin-line converging optics system1000 . . . thin linear illumination region 11 . . . detection opticsunit 11 a,11 b,11 c . . . detection unit 111 a,111 b,111 c . . .objective lens 112 a,112 b,112 c . . . spatial filter 113 a,113 b,113 c. . . polarization filter 114 a,114 b,114 c . . . imaging lens 115 a,115b,115 c . . . image sensor 12 . . . data-processing unit 121 a,121 b,121c . . . signal-processing unit 122 . . . image-processing unit 13 . . .stage unit

What is claimed is:
 1. A defect inspection method, comprising:irradiating a linear region on a surface-patterned sample mounted on atable which moves in a plane, with illumination light from an inclineddirection relative to a direction of a line normal to the sample;detecting in each of a plurality of directions an image of scatteredlight originating from the sample irradiated with the illuminationlight; and detecting a defect on the sample by processing signalsobtained by the detection of the images of the scattered light; whereinthe step of detecting the scattered light image in the plural directionsis performed through elliptical lenses in which elevation angles of theoptical axes thereof are different from each other, within one planeperpendicular to a plane formed by the normal to the surface of thetable on which to mount the sample and the longitudinal direction of thelinear region irradiated with the irradiation light, the ellipticallenses being formed of circular lenses having left and right portionsthereof cut.
 2. The defect inspection method according to claim 1,wherein, in one plane perpendicular to a plane formed by the normal tothe surface of the table on which to mount the sample and thelongitudinal direction of the linear region irradiated with theirradiation light, the image detection of the scattered light from theplurality of directions is conducted in a plurality of symmetricaldirections relative to the normal line.
 3. The defect inspection methodaccording to claim 1, wherein the image detection of the scattered lightfrom the plurality of directions is conducted in a plurality ofdirections including a direction of the line normal to the surface onwhich the sample is mounted.
 4. The defect inspection method accordingto claim 1, further comprising: detecting the images of thesample-scattered light from the plurality of directions by calibrating adegree of matching in quality of each of the images, in response to atemperature environment and atmospheric pressure environment of aposition at which the sample is inspected.
 5. The defect inspectionmethod according to claim 4, wherein the degree of matching in qualityof each of the images is calibrated by controlling a spacing between animaging lens used to form the images of the scattered light, and imagesensors that detect the scattered-light images formed by the imaginglens.
 6. The defect inspection method according to claim 4, wherein thedegree of matching in quality of each of the images is calibrated bymoving at least one of a plurality of lenses of a lens system that formthe images of the scattered light, in relative fashion with respect toan optical-axis direction of the lens system.
 7. The defect inspectionmethod according to claim 4, wherein the degree of matching in qualityof each of the images is calibrated by shifting a wavelength of theillumination light.
 8. A defect inspection device, comprising: a tableunit adapted to move in a plane with a surface-patterned sample mountedon the table unit; an illumination optics unit that irradiates a linearregion on the sample mounted on the table unit, with illumination lightfrom an inclined direction relative to a direction of a line normal tothe patterned surface of the sample; a detection optics unit thatdetects an image of scattered light originating from the sampleirradiated with the illumination light by the illumination optics unit;and an image-processing unit that detects a defect on the sample byprocessing a signal obtained from the image of the scattered light thatthe detection optics unit has detected; wherein the detection opticsunit includes a plurality of detection optical systems arranged so thatelliptical lenses in which elevation angles of the optical axes thereofare different from each other are arranged within one planeperpendicular to a plane formed by the normal to the surface of thetable unit on which to mount the sample and the longitudinal directionof the linear region irradiated with the irradiation light by theillumination optics unit, the detection optical systems each includingan objective lens that is the elliptical lenses formed of circularlenses having left and right portions thereof cut.
 9. The defectinspection device according to claim 8, wherein the plurality ofdetection optics systems in the detection optics unit are arrangedsymmetrically with respect to the line normal to that surface of thetable unit where the sample is mounted, within one plane perpendicularto a plane formed by the normal to the surface of the table unit onwhich to mount the sample and the longitudinal direction of the linearregion irradiated with the irradiation light by the illumination opticsunit.
 10. The defect inspection device according to claim 8, wherein oneof the plurality of detection optical systems in the detection opticsunit is disposed along the line normal to that surface of the table unitwhere the sample is mounted.
 11. The defect inspection device accordingto claim 8, wherein the detection optics unit includes an image qualitycalibrating mechanism for calibrating, in response to a temperatureenvironment and atmospheric pressure environment of an inspectionposition, a degree of matching in quality of each of thesample-scattered light images detected by the plurality of detectionoptical systems.
 12. The defect inspection device according to claim 11,wherein: the plurality of detection optical systems in the detectionoptics unit each include an imaging lens and an image sensor; and theimage quality calibrating mechanism is a mechanism that moves the imagesensor in relative fashion with respect to an optical-axis direction ofthe imaging lens.
 13. The defect inspection device according to claim11, wherein: the plurality of detection optical systems in the detectionoptics unit each include a lens system formed from a combination of aplurality of lenses, and an image sensor; and the image qualitycalibrating mechanism is a mechanism that moves at least one of theplurality of lenses of the lens system in relative fashion with respectto an optical-axis direction of the lens system.
 14. The defectinspection device according to claim 11, wherein: the plurality ofdetection optical systems in the detection optics unit each include alens system formed from a combination of a plurality of lenses, and animage sensor; and the image quality calibrating mechanism is a mechanismthat shifts a light source wavelength of the illumination optics system.